Technical Field
The present invention relates to a water treatment process that includes oxidizing organic contaminants under exposure to ultraviolet light, especially as it relates to the removal of traces of methyl tertiary-butyl ether (MTBE).
Description of the Related Art
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.
The chemical water pollutant, methyl tertiary butyl ether (MTBE), is a well-known groundwater contaminant which mainly originates from petrochemical and transportation industries. Higher production levels and widespread use of MTBE make it likely to be present in groundwater sources. Leakage from underground storage tanks and pipelines, spills, contaminated sites, releases from manufacturing, storage sites, and at gasoline filling stations account for the major sources of environmental contamination.
Contamination of drinking water with MTBE has raised considerable concern among health officials and water suppliers. The US Environmental Protection Agency considers MTBE to be a potential human carcinogen, and set an advisory level of 20-40 μg/L. See U.S. Environmental Protection Agency, (1997b). Drinking Water Advisory: Consumer Acceptability Advice and Health Effects Analysis on Methyl Tertiary-Butyl Ether (MTBE). Washington, D.C.: U.S. Environmental Protection Agency, Office of Water, EPA-822-F-97-009. However, remediation of MTBE from water is challenging and expensive due to MTBE's high solubility in water (50,000 mg/l), low biodegradability, low Henry's constant, very low affinity for common adsorbents, high mobility, and public health concern. See McCarthy & Tiemann, “MTBE in gasoline: clean air and drinking water issues” Congressional Research Service Reports, 2006; J. Reuter, Allen, & Goldman, “Methyl tert-butyl ether in surface drinking water supplies,” Health and environmental assessment of MTBE, UC Toxics Research and Teaching Program, UC Davis 3 (1998); J. E. Reuter et al., 1998, “Concentrations, Sources, and Fate of the Gasoline Oxygenate Methyl tert-Butyl Ether (MTBE) in a Multiple-Use Lake,” Environmental Science & Technology, 32(23), 3666-3672, each incorporated herein by reference in their entirety. Advanced oxidation processes (AOPs) have been acknowledged as promising treatment technologies for water contaminated with MTBE. AOPs have shown great potential in removing organic contaminants at low and high levels in groundwater, municipal and industrial wastewater. See Kavanaugh, Michael, and Z. Chowdhury, “Removal of MTBE with advanced oxidation processes,” Iwa Publishing, 2004, incorporated herein by reference in its entirety. Ultraviolet (UV)-driven AOPs are primarily based on the generation of powerful oxidizing species, such as the hydroxyl radical (OH.). These processes make use of hydroxyl radicals (OH.) to oxidize all organic pollutants present in water into carbon dioxide and water.
The chlorine based photochemical oxidation or UV/Cl2 is one type of AOP used to degrade organic contaminants. The photo-chemistry of the UV/chlorine process predominantly generates hydroxyl (OH) radical in addition to chlorine radical. The pseudo-first-order rate constant for the photolysis of HOCl and OCl− at 330 nm (sunlight exposure) was reported as 2×10−4 s−1 and 1.2×10−3 s−1, respectively. Sodium hypochlorite (NaOCl) has been also used to oxidize secondary (2°) alcohols to ketones.
At favorable pH, free chlorine is available in water as an aqueous solution. The photochemistry of UV/Cl2 AOP is described by the reactions below. See Jin, Jing, Mohamed Gamal El-Din, and James R. Bolton. “Assessment of the UV/Chlorine process as an advanced oxidation process.” water research 45.4 (2011): 1890-1896; Wang, Ding, James R. Bolton, and Ron Hofmann. “Medium pressure UV combined with chlorine advanced oxidation for trichloroethylene destruction in a model water.” water research 46.15 (2012): 4677-4686, each incorporated herein by reference in their entirety.Cl2+H2O→OCl+HCl  (1)OCl→H++OCl−(equilibrium with pKa=7.6 at 20° C.  (2)OCl+UV photons→.OH+Cl.  (3)OCl−+UV photons→.O−+Cl.  (4).O−+H2O→.OH+OH−  (5)
The availability of free chlorine is dependent on the pH of the solution, as discussed by several studies. In conventional processes the percent availability of free chlorine species at room temperature may be 99.7% HOCl, 52.3% HOCl+47.7% OCl−, and 99.6% OCl− at pH 5, 7.5 and 10, respectively. See Chan, Po Yee, Mohamed Gamal El-Din, and James R. Bolton. “A solar-driven UV/Chlorine advanced oxidation process.” water research 46.17 (2012): 5672-5682; Feng, Yangang, Daniel W. Smith, and James R. Bolton. “Photolysis of aqueous free chlorine species (HOCl and OCl) with 254 nm ultraviolet light.” Journal of Environmental Engineering and Science 6.3 (2007): 277-284; Mofidi, A. A., Min, J. H., Palencia, L. S., & Coffey, B. M. (2002). Task 2.1: Advanced Oxidation Processes and UV Photolysis for Treatment of Drinking Water Submitted by: Sun Liang, James F. Green Metropolitan Water District of Southern California La Verne, Calif. Submitted to: California Energy Commission, (January); Nowell, Lisa H., and Jürg Hoigné. “Photolysis of aqueous chlorine at sunlight and ultraviolet wavelengths—I. Degradation rates.” Water Research 26.5 (1992): 593-598.; Watts, Michael J., and Karl G. Linden. “Chlorine photolysis and subsequent OH radical production during UV treatment of chlorinated water.” Water Research 41.13 (2007): 2871-2878.; Weng, ShihChi, Jing Li, and Ernest R. Blatchley. “Effects of UV 254 irradiation on residual chlorine and DBPs in chlorination of model organic-N precursors in swimming pools.” water research 46.8 (2012): 2674-2682.; Rick Bond, P. E., B. & V. Advanced Oxidation Processes: White's Handbook of Chlorination and Alternative Disinfectants, Fifth Edition, (2010) 976-1002.; White, G. C., & International, B. and V. (2010). Chlorination and Alternative Disinfectants (5th Edition). Wiley, each incorporated herein by reference in their entirety.
Only a few studies investigated the use of aqueous chlorine as the chemical oxidant for UV-driven AOP as an alternative to other chemical oxidants like hydrogen peroxide, and ozone. Similar to UV/H2O2, AOPs, UV-induced chlorine AOPs (UV/Cl2 AOP) produce hydroxyl and other radicals when water dosed with aqueous chlorine in the form of hypochlorous acid (HOCl) or hypochlorite ions (ClO−) and exposed to UV light. Hypochlorous acid (HOCl) has higher UV absorbance and a lower scavenging rate than H2O2. In contrast, ClO— scavenges OH radicals about four orders of magnitude faster than HOCl or H2O2, indicating that UV/Cl2 AOPs are generally more efficient at lower water pH. UV/Cl2 was efficiently able to degrade trichloroethylene, Methylisoborneol in water than UV/H2O2 process. See Rosenfeldt, Erik, et al. “Tech Talk—Comparison of UV-mediated Advanced Oxidation (PDF).” Journal-American Water Works Association 105.7 (2013): 29-33.; Wang, Ding, James R. Bolton, and Ron Hofmann. “Medium pressure UV combined with chlorine advanced oxidation for trichloroethylene destruction in a model water.” Water Research 46.15 (2012): 4677-4686., each incorporated herein by reference in their entirety. A solar-driven UV/chlorine AOP was able to degrade methylene blue (MB) and cyclohexanoic acid (CHA) in water. Another study reported that UV/Chlorine AOP was also able to degrade emerging water contaminants with considerable energy reduction. A few studies investigated the use of UV/chlorine AOP for the degradation of organic contaminants in water and wastewater. The quantum yield of OH radical production from HOCl at a wavelength of 254 nm was found to be 1.4 mol·s−1 greater than that of hydrogen peroxide (1.0 mol·s−1) whereas other studies found the quantum yields of HOCl and OCl− are 1.0±0.1 and 0.9±0.1, respectively. Still, others have reported molar absorption coefficient of 155 and 121 ε254/M−1 cm−1 for HOCl and OCl−, respectively. See Sichel, C., C. Garcia, and K. Andre. “Feasibility studies: UV/chlorine advanced oxidation treatment for the removal of emerging contaminants.” Water Research 45.19 (2011): 6371-6380; Watts, Michael J., and Karl G. Linden, “Chlorine photolysis and subsequent OH radical production during UV treatment of chlorinated water.” Water Research 41.13 (2007): 2871-28781; Nowell, Lisa H., and Juirg Hoigné. “Photolysis of aqueous chlorine at sunlight and ultraviolet wavelengths—II. Hydroxyl radical production.” Water Research 26.5 (1992): 599-605. each incorporated herein by reference in their entirety.
In view of the forgoing, one objective of the present disclosure is to provide a process for the removal of MTBE and associated organic compounds from aqueous solution using a UV/chlorine process, optionally, under a continuous flow regime for MTBE and associated organic compound removal from aqueous solutions by UV/chlorine process.